1. Field of the Invention
This invention relates generally to earth materials and the environment. More specifically, this invention relates to a process for treating rock heaps to prevent oxidized species release, including acid drainage, therefrom.
2. Related Art
Acid drainage from closed mines and waste heaps has been an important environmental problem. Prior art methods to alleviate this problem include collecting and treating acidic water discharged from the polluting source. Also, other treatment methods have attempted to prevent the acid drainage from the source in the first place.
U.S. Pat. No. 4,861,482, issued Aug. 29, 1989 , (Frankenberger et al.) discloses removing selenium from soil and water by adding nutrients, including cobalt, zinc and nickel ions, for microbes in the soil or water, resulting in the formation of volatile alkylselenides.
U.S. Pat. No. 5,387,271, issued Feb. 2, 1995, (Crawford et al.) discloses biodegrading nitroaromatics in soil and water with a first fermentation operation performed by an inoculum of aerobic and/or facultative microorganisms fed by an added carbohydrate nutrient. This first operation is followed by a second anaerobic operation performed by an inoculum of mixed anaerobic microorganisms fed by the remaining carbohydrate nutrient.
U.S. Pat. No. 5,399,048 (Walker), issued Mar. 21, 1995, discloses capping an exposed rock heap surface with an impermeable coating containing seeds and/or a coloring additive for esthetic purposes.
U.S. Pat. No. 5,616,251 (Batarseh), issued Apr. 1, 1997, discloses treating material causing acid mine drainage with a liquid containing a Lewis base group (Mxe2x80x94) attached to a hydrophobic group (xe2x80x94R) for example, water-soluble alkylated phosphate. The Lewis base group component reacts with the metal in the acid-causing material.
U.S. Pat. No. 5,632,715 and 5,710,361 (Harrington et al.), issued Mary 27, 1997 and Jan. 1, 1998, respectively, disclose in-situ immobilization of metals in waste heaps by injecting a nutrient into a passageway into the heap, the nutrient feeding sulfide-producing microorganisms also provided to the heap to create less-soluble metal sulfides in the heap.
Still, there exists a need to prevent acid drainage from rock heaps indefinitely in a self-sustaining manner. This invention addresses that need.
The invention is a process for treating earth materials, such as rock heaps, for example, to prevent acid drainage therefrom. According to the invention, a first step comprises displacing and/or depleting the oxygen concentration in the gas-phase of the rock heap. Preferably, the rock heap gas-phase oxygen concentration is reduced in this step enough to prevent oxidative reactions that cause acid and soluble metal formation. Preferred methods of this displacement and/or depletion include: displacement by another gas, which may be called xe2x80x9cphysical depletionxe2x80x9d; displacement/depletion by chemical reaction(s), which may be called xe2x80x9cchemical depletion,xe2x80x9d in which oxygen is displaced by gas reaction products or is consumed by the reaction(s); or displacement/depletion by the reactions of microbiological activity and by the gasses produced by that activity, which may be called xe2x80x9cbiological depletionxe2x80x9d.
Optionally, selected microbial nutrients that favor reductive reactions rather than oxidative reactions may be added to the rock heap. This way, in rock heaps containing sulfide materials, carbon oxidation is thermodynamically favored over sulfide oxidation, causing gas-phase depletion of oxygen via carbon dioxide formation. In addition, in-situ cyanide and nitrate degradation reactions are accelerated.
Another aspect of the prevent invention is the lessening or eliminating of the xe2x80x9cchimney effectxe2x80x9d that typically occurs in a prior art rock pile, and the consequent lessening/eliminating of oxygen infiltration (xe2x80x9cin-flowxe2x80x9d) into the rock pile. The xe2x80x9cchimney effectxe2x80x9d in a prior art rock pile is the flow of ambient air into the pile at its sides near its bottom, and then through the pile and out its top. This bottom-to-top air flow is caused by oxidative chemical/biochemical reactions occurring in the prior art rock pile, which reactions are exothermic, causing the pile to heat up. This heating effect reduces the density of the gases in the rock pile, resulting in their floating up and out of the pile. Also, because the chemical/biochemical reactions occurring in the prior art pile are oxidative, the relative composition of the gas phase changes, resulting in gases of less average density. This lighter gas composition effect also contributes to the prior art xe2x80x9cchimney effect.xe2x80x9d
In the present invention, formation of lower oxygen concentration conditions in the rock heap increases the density of the rock heap gas-phase, blocking the xe2x80x9cchimney effect.xe2x80x9dThe formation of carbon dioxide gas (CO2) from oxygen gas (O2) increases the average density of the rock pile gas-phase. This increase in average density reduces or prevents updraft of rock pile gases and, therefore, infiltration by oxygen flowing into the rock pile from the lower sides of the pile. Thus, preferred embodiments of the present invention reduce the oxygen concentration in the pile by displacement and/or depletion and also by preventing infiltration of oxygen.
According to an additional step of the instant invention, the gas-phase oxygen concentration in the heap is maintained at a low level. For example, additional selected nutrients are provided to the rock heap to protect against future oxygen infiltration. Preferably, to obtain this maintenance condition indefinitely, the surface of the rock heap is covered with a thin layer of soil which is planted with vegetation. The natural biological cycle of the vegetation on the surface of the rock heap provides the microbial nutrients necessary to indefinitely maintain the low oxygen con centration conditions in the heap.